A simple photolithography method was developed for patterning organic field effect transistors (OFETs) prepared from blends of poly(3-hexylthiophene), P3HT, and [6,6]-phenyl-C61-butyric acid methyl ester, PCBM.(1) This Photo-induced and Resist-free Imprint patterning (PRI) technique allows also the single solution step production of organic CMOS circuits.(2) It consists of two subsequent processing steps: 1) exposure: photo-irradiation of the P3HT:PCBM blend by visible laser light and 2) development: rinsing of the film in an organic solvent mixture that is selective for PCBM. As a result, two electronically different materials are obtained, i.e. the exposed and developed (ED) material, and the unexposed and developed (UD) material. The method is based on the modification of the PCBM component in the irradiated area, which becomes effectively insoluble in the solvent mixture, while the PCBM in the non-irradiated area is removed during development. Therefore, we expect that the UD material is pure P3HT, a hypothesis that is confirmed by the p-type conductivity of the ED region. Near-Edge X-ray Absorption Fine Structure spectroscopy (NEXAFS) was used to determine the surface composition of these films. C K-edge NEXAFS spectra of pristine, photo-exposed, and developed blend films, as well as films of the pure components were measured at the synchrotron facility MAX-lab in Lund, Sweden. The spectra for P3HT and PCBM are significantly different and the components can be clearly distinguished in the blend spectra. From the relative intensities of the P3HT and PCBM peaks, the actual blend composition can be estimated, both on the surface, using partial electron yield (PEY), and deeper in the sub-surface region of the film, using total electron yield (TEY). From the similarity of the spectra of the UD blend sample and the pure P3HT sample, we conclude that the remaining material after washing the pristine blend is indeed P3HT, and the ED blend sample retains its two-component character. The surface composition of the blend films is significantly more polymer-rich than the bulk blend ratio used to prepare the film. Both for the pristine blend and the photo-exposed blend differences are observed between the PEY and TEY spectra, indicating the existence of a polymer-enriched surface. Such gradients in thin films of P3HT:PCBM blends have been observed by others using variable-angle spectroscopic ellipsometry,(3) NEXAFS,(4) and neutron reflectometry,(5) and also in other polymer:PCBM blends by dynamic secondary ion mass spectrometry (d-SIMS).(6)
References (1) Dzwilewski, A.; Wagberg, T.; Edman, L. J. Am. Chem.Soc. 2009, 131, 4006. (2) Dzwilewski, A.; Matyba, P.; Edman, L. J. Phys. Chem. B 2010, 114, 135. (3) Campoy-Quiles, M., et al., Nature Materials 2008, 7,158-164 (4) Germack,D.S. et al., Appl. Phys. Lett. 2009, 94, 233303. (5) Kiel, J.W. et al., Soft Matter 2010, 6, 641-646. (6) Björström, C.M. et al, J. Phys.: Condens. Matter 2005, 17, L529-L534.